March 2002 On-Post Quarterly GW Report

March 2002 On-Post Quarterly Groundwater Monitoring Report

Section 2 - Basewide Flow Direction and Gradient

A groundwater potentiometric surface map generated from the March 2002 groundwater elevations is shown in Figure 2-1. Among the cluster wells CS-MW6-LGR, CS-MW7-LGR, CS-MW8-LGR, CS-MW9-LGR, and CS-MW10-LGR, only the water level measurements from the Lower Glen Rose were used in creating the potentiometric surface map. The March 2002 potentiometric surface map indicates a variety of flow directions. The dominant groundwater gradient appears to be to the southeast at 0.0086 feet/feet, which is consistent with the last five events of groundwater monitoring conducted, since March 2001. Groundwater flow directions and gradients during past monitoring events are provided in Table 4 of the Introduction to the Quarterly Groundwater Monitoring Program (Parsons 2001) located in Volume 5, Groundwater, for comparison.

The overall observation from the March 2002 potentiometric surface map (Figure 2-1) is that CSSA wells exhibited a wide range of groundwater elevations. The water level measurements suggest that groundwater elevations are somewhat higher in the central and northwestern portions of CSSA. Overall, the groundwater elevations are generally lower towards the south with Well CS-1 having the lowest groundwater elevation of all measured wells.

There were exceptions to the general south-southeast decline in groundwater elevations. Well CS-MW4-LGR in the central portion of CSSA had one of the highest groundwater elevations measured at 1128.38 feet above mean sea level (MSL) (Figure 2-1). In the northwestern corner of CSSA, Well CS-H had a groundwater elevation measured at 1181.34 feet MSL, which is the highest of the measured water levels. In this localized area at the northwestern corner, the groundwater gradient appears to be to the northwest, toward off-post well FO-20. The two wells CS-MW4-LGR and CS-H have consistently exhibited the highest two groundwater elevations measured, since the June 2001 sampling event. These two localized areas consistently report a different groundwater gradient direction from the overall south-southeast decline in water elevations.

The groundwater gradient presented in Figure 2-1 incorporates measured groundwater elevations from the Lower Glen Rose wells and those wells with open borehole completions (CS-1, CS-9, CS-10, CS-11, CS-D, and CS-16). In the area around Building 90, in the southwest corner of CSSA, potentiometric surface maps were created using March 2002 groundwater elevations from wells screened in the Lower Glen Rose and Cow Creek units (Figure 2-2 and Figure 2-3, respectively). The September 2001 event was the first event in which local potentiometric maps were prepared for the area around Building 90. The Lower Glen Rose potentiometric surface map for March 2002 indicates groundwater elevations slope to the southeast, locally, at Building 90. This direction is a minor change from the December 2001, groundwater elevations that indicated a gradient sloped to the southwest in the Lower Glen Rose. The December 2001 gradient was a complete reversal from the apparent flow direction to the north observed in September 2001. The Cow Creek potentiometric surface map indicates that the March 2002 groundwater gradient slopes toward the south which is a reversal of the flow direction observed in the Cow Creek in September 2001 and December 2001, when elevations decreased toward the north.

As shown in Figure 2-1, water levels at CSSA vary greatly. This variability is likely associated with various factors:

	Differences in well completion depths and formations penetrated;
	Differences in recharge rates due to increased secondary porosity associated with the Salado Creek floodplain;
	Differences in recharge rates due to increased secondary porosity associated with the fault zone;
	Unknown pumping rates from public and private water supply wells located off-post but near the CSSA boundary; and
	Locations of major faults or fractures.

Most potentiometric surface maps prepared for CSSA are based on water levels from wells with different completion depths, with the exception of figure 2-2 and figure 2-3. Additional information concerning this issue is included in the introduction to the quarterly groundwater monitoring program (Volume 5, Groundwater). Because several wells depicted on Figure 2-1 are open to multiple water-bearing zones, potentiometric surface maps should be considered qualitatively. The differences in water levels across CSSA may stem from differences in the various wells� completions. Wells CS-D, CS-2, CS-4, CS-MW1-LGR and CS-MW2-LGR are open-hole completions in the Lower Glen Rose, or the Lower Glen Rose and the upper portion of the Bexar Shale. Well CS-16 is open through the Lower Glen Rose, the Bexar Shale and the Cow Creek. Therefore, water level measurements obtained from some wells (i.e., CS-16 and CS-1) represent water levels measured from up to three different formations. Wells completed in the Lower Glen Rose only represent the water level for that formation. Interpretation of the data for the overall potentiometric surface map is complicated by these well completion differences. As more wells are completed in the different formations, use of well data from wells screened through multiple formations can be reduced or eliminated in future water level and gradient determinations.

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